Warmer Temperatures Push Malaria to Higher Elevations

Researchers are reporting the first hard evidence that malaria, a mosquito-borne disease that infects more than 300 million people each year, creeps to higher elevations during warmer years and falls back down to lower altitudes when temperatures cool.

"Traditionally, we think of malaria as a disease with limited prevalence in highland regions, but we are now seeing a shift due to climate change. Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new high-altitude areas. And because these populations lack protective immunity, they will be particularly vulnerable for severe morbidity and mortality," said Menno Bouma, of the London School of Hygiene and Tropical Medicine.

The study predicts that future climate warming will result in a significant increase in malaria cases in densely populated regions of Africa and South America, unless disease monitoring and control efforts are boosted and sustained.

It had been predicted that malaria could be sensitive to climate change, because both the Plasmodium parasites that cause it and the Anopheles mosquitoes that spread it thrive as temperatures warm. But some of the assumptions behind the predictions were criticized. Recently, some researchers have argued that improved socioeconomic conditions and more aggressive mosquito-control efforts may exert a far greater influence over the extent and intensity of malaria than climatic factors.

Since a detailed analysis of regional records to determine how the spatial distribution of malaria cases has changed in response to year-to-year temperature variations has been missing in this debate, Bouma and his colleagues looked for evidence of a changing spatial distribution of malaria with varying temperature in the highlands of Ethiopia and Colombia.

These areas have densely populated highlands and have historically provided havens from the disease. By excluding other variables that influence malaria case numbers, such as mosquito-control programs, resistance to anti-malarial drugs and fluctuations in rainfall amounts, the researchers found that the median altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler years. Their relatively simple analysis yielded a clear, unambiguous signal that can only be explained by temperature changes.

The researchers did note, however, that their findings underscore the size of the problem and emphasized the need for sustained intervention efforts in these regions, particularly in Africa.